1. Field of the Invention
The present invention relates to a start-up circuit and motor driving IC, and more particularly, to a start-up circuit and motor driving IC capable of determining an operating mode of the motor driving IC by itself without an enable pin, to adjust an activation signal to perform activation.
2. Description of the Prior Art
As the development of computer technology in recent years, the heat generated from a central processing unit (CPU) increases as the frequency of the CPU increases. Therefore, the need for heat-dissipation becomes important. The main method for heat-dissipation is still heat-dissipating fans. There are voltage control and pulse width modulation (PWM) control methods for motor driving ICs of heat-dissipating fans used in CPUs.
Please refer to FIG. 1A and FIG. 1B, which are schematic diagrams of conventional motor driving ICs 10 and 12 having a voltage control mode and a PWM control mode, respectively. Noticeably, the motor driving ICs 10 and 12 can be a same motor driving IC in practice as long as control modes of the motor driving IC are switched via adjusting input signals of a system voltage pin VCC_P and a PWM pin PWM_P.
In detail, as shown in FIG. 1A, for the voltage control mode of the motor driving IC 10, a driving current IL at the output pins OUT1_P and OUT2_P can be changed via adjusting a system voltage VCC at a system voltage pin VCC_P, so as to change rotation speeds of a motor 102 and the corresponding fan.
On the other hand, as shown in FIG. 1B, for the PWM control mode of the motor driving IC 12, the system voltage VCC at a system voltage pin VCC_P is fixed, and then the driving current IL outputted by the output pins OUT1_P and OUT2_P can be changed by adjusting the duty of a PWM signal PWMS at a PWM pin PWM_P, so as to change the rotation speeds of the motor 102 and the corresponding fan.
For example, please refer to FIG. 1C, which is a schematic diagram of a driving circuit 104 of the motor driving IC 12 shown in FIG. 1B. As shown in FIG. 1B, the motor driving IC 12 can utilize a PWM output signal PWMout, which is equivalent to the PWM signal PWMS, to control the on/off of an upper gate switch 106 and a lower gate switch 108 of the driving circuit 104, so as to change the driving current IL of the driving motor 102, and thus change the rotation speeds of the motor 102 and the corresponding fan as well. Noticeably, the motor driving IC 10 can also comprise the driving circuit 104 shown in FIG. 1C to drive the motor 102, where the motor driving IC 10 can be seen as the motor driving IC 12 with the PWM pin PWM_P not coupled to the PWM signal PWMS, i.e. floating, and thus a duty of the inputted PWM signal PWMS is equivalent to a full duty.
However, for the PWM driving IC 12, if the duty of the PWM signal PWMS is too small, and thus the generated driving current IL is too small to overcome the static friction, the motor 102 can not be activated. In such a situation, the conventional PWM driving IC 12 utilizes the PWM output signal PWMout with greater duty, e.g. a duty of 50%, to accordingly generate the greater driving current IL first, so as to activate the motor 102 compulsively. After the motor 102 starts rotating, the PWM driving IC 12 returns to utilize the PWM signal PWMS with lesser duty as the PWM output signal PWMout.
As a result, please refer to FIG. 1D, which is a schematic diagram of that the rotation speeds of the motor 102 shown in FIG. 1B with respect to different duty of the PWM signal PWMS whether the conventional compulsive activation mechanism is applied or not. As shown in FIG. 1D, without the compulsive activation mechanism, e.g. a duty of 50%, the motor 102 can be activated by the PWM signal PWMS with a duty about 20%. Oppositely, with the compulsive activation mechanism, the minimum duty to activate the motor 102 can be improved to be 10%.
In practice, the motor driving ICs 10 and 12 can be the same motor driving IC by means of adjusting input signals of a system voltage pin VCC_P and a PWM pin PWM_P to switch control modes of the motor driving IC. If the mentioned compulsive activation mechanism is built in the motor driving ICs 10 and 12, the motor driving IC 10 generates the driving current IL with the PWM output signal PWMout without a full duty, e.g. a duty of 50%, to activate the motor 102. As a result, under the circumstances that the motor driving IC 10 is operating in the voltage control mode and the system voltage VCC is at a low voltage level, the motor driving IC 10 may not be able to overcome the static friction to activate the motor 102.
In such a situation, please refer to FIG. 1E, which is a schematic diagram of a conventional motor driving IC 14 further comprising an enable pin EN. As shown in FIG. 1E, if a signal of the enable pin EN is at a low voltage level, the motor driving IC 14 does not perform the compulsive activation mechanism and thus the voltage control mode is adapted. If the signal of the enable pin EN is at a high voltage level, the motor driving IC 14 performs the compulsive activation mechanism and thus the PWM control mode is adapted.
However, since the pins of a general motor driving IC are limited, the conventional method of using the enable pin EN to control whether to enable the compulsive activation mechanism results in limited functions of the motor driving IC. Therefore, there is a need to improve the prior art.